Tossed aside by investors and future retirees for decades, index investing has made major strides in recent years. Its advocates – spawning from the legendary Jack Bogle – are right when they argue that low cost index investing has trounced the active managers. This in part derives from the high fees charged by many active funds. However, arguably one source of index investing’s success was and is widespread active investing to sift through all the information. Large masses of active investors seem likely to make index holdings more efficiently priced. Yet – if the masses move to indexes in great enough numbers – could markets slip up? Perhaps that might make some more room for active investors to start winning.